The present invention generally relates to a sheet handling apparatus and more particularly, to a sheet feeding arrangement to be used for positively copying and transporting sheets, one by one in a copying apparatus, electronic file facsimile device, printing press or the like.
Conventionally, there have been proposed various document feeding apparatuses arranged to transport original documents in a sheet form picked up sheet by sheet, up to a predetermined position and, for example, after subjecting the sheet to image processing, adapted to discharge the sheet thus processed onto a discharge tray.
In FIG. 4, showing the most common example of such known feeding apparatus, a sheet P sent out by a sheet pick-up device 110 from a stack of sheets placed on a sheet feeding table 114, is transported to a predetermined sheet processing position by a transport device 120 including an endless belt 126 movably supported by rollers 125. The sheet P is depressed by the belt 126, i.e., through the utilization of friction between said belt 126 and the sheet P and also the friction between said sheet P and the surface of a transparent platform, e.g., a platen glass 141 for the image processing section of an image processing device 140 on which the feeding apparatus is mounted. At a predetermined position, the sheet P transported by said belt 126 is subjected to image processing under the state where said sheet is pressed against the platen glass 141 as stated earlier for subsequent discharge of said sheet P onto a discharge tray 130. In FIG. 4, the image processing device 140 includes, for example, an exposure device 56 for the sheet P adapted to be displaced from a position indicated by a solid line to a position shown by a dashed line in a direction of an arrow f.
In the above known arrangement, however, since the transport is effected through utilization of friction, there are various disadvantages such as a tendency to damage the sheets, soiling a the sheet surface, tendency to cause slippage with respect to the sheets having a low surface coefficient of friction, deformation of the belt during transportation, fluctuation of tension, zigzag movement due to an imbalance in the roller configuration, and a tendency to electrically charge the sheets due to a large depressing force, etc.
In order to eliminate such disadvantages as referred to above, there has also conventionally been proposed an apparatus which is arranged to attract the sheet through a utilization of air-suction so as to transport said sheet thus attracted as illustrated in FIG. 5.
The sheet feeding apparatus utilizing the air-suction as shown in FIG. 5 further includs a sheet attracting section 150 formed with air suction holes in the bottom surface, and connected with a suction air causing 160 in which a negative pressure chamber and a negative pressure generating device are provided. The sheet attracting section 150 and the suction air causing 160 are disposed within the loop of the belt 126' so as to attract one side face of the sheet P to the sheet attracting section 150, thereby transporting the sheet as it is attracted at one side face through movement of the belt 126' or rotation of a frictional member such as a roller or the like (not particularly shown). In FIG. 5, like parts in FIG. 4 are designated by like reference numerals for brevity of description.
However, the conventional sheet feeding arrangements as described so far are disadvantageous, particularly when thin sheets are to be dealt with. Also, adjustment of its suction force is troublesome, and even thin and thick sheets are present in a mixed state, transportation thereof is difficult due to the above reason. With respect to a sheet having a high transparency, not only the soiling of the belt, but also that of the suction holes is undesirably transferred. Similarly, regarding the occurrence of the zigzag movement, no fundamental countermeasures are available with respect to the former arrangement which does not utilize the suction air. Meanwhile, the latter arrangement which utilizes air suction leads to a large size of the apparatus due to the necessity for the suction air casing, etc.
Furthermore, in any of the above known arrangements, it is difficult to effect the sheet discharge in a stable state due to undesirable electrical charging of the sheet.
Additionally, in the case where a sheet having images on both side faces is to be dealt with, a very complicated transport conveyance must be provided if the above arrangements are adopted, thus increasing the frequency of problems during the transportation.